Tires may be prepared with treads comprised of diene-based rubber compositions which contain reinforcing filler as a combination of rubber reinforcing carbon black and precipitated silica. The precipitated silica may be used together with a silica coupler compound to couple the precipitated silica to diene-based elastomers contained in the rubber composition.
For intended heavy duty service, particularly for a truck tire, it is sometimes desired to promote heavy duty performance physical properties for its tread such as, for example, a combination of abrasion resistance and crack growth resistance properties while providing a suitable stiffness property.
An increase in such physical properties of a rubber composition may often be expected to increase its hysteresis property which, in turn, may be expected to promote an undesirable increase in internal heat generation for the tire tread during the tire's heavy duty service. An increase in such hysteresis property may be indicated, for example, by a decrease in the rubber composition's rebound property, particularly its hot (e.g. 100° C.) rebound property, and/or an increase in its tangent delta (tan delta) property.
Here, a challenge is presented to promote such combination of physical properties with a minimal increase, if any, of a hysteresis property of the tread rubber composition and thereby desirably substantially maintaining its hysteresis.
It is further desired to promote such physical properties for the tread rubber composition without significantly increasing, and desirably substantially maintaining, its uncured processing viscosity to thereby promote satisfactory processing (e.g. mixing, extrusion and calendering) of the rubber composition.
For such challenges it is desired to evaluate providing the tread rubber as a composite of a blend of rubber phases individually comprised of combinations of diene-based elastomer(s) and reinforcing fillers selected from rubber reinforcing carbon black and precipitated silica.
It is proposed for the overall tread rubber to contain diene-based elastomers comprised of cis 1,4-polybutadiene and natural rubber (natural cis 1,4-polyisoprene) with reinforcing filler comprised of a combination of rubber reinforcing carbon black and precipitated silica with silica coupling agent for the precipitated silica.
It is further proposed to provide the tread rubber composition as a composite of a blend of a first phase comprised of cis 1,4-polybutadiene rubber and reinforcing filler comprised of rubber reinforcing carbon black and a second phase comprised of natural rubber (natural cis 1,4-polyisoprene) and reinforcing filler comprised of precipitated silica.
For such phased rubber blend, it is proposed to provide a first phase comprised of a masterbatch of cis 1,4-polybutadiene rubber and reinforcing filler comprised of rubber reinforcing carbon black together with metal oxide comprised of zinc oxide and added fatty acid and to provide a second phase comprised of a masterbatch of natural rubber and reinforcing filler comprised of precipitated silica without the zinc oxide and added fatty acid.
The zinc oxide is to be selectively provided in the first phase (the polybutadiene rubber with carbon black reinforcing filler) to avoid pre-mature reaction of the zinc oxide with the precipitated silica and silica coupler and to thereby promote a more delayed interaction of the zinc oxide with the precipitated silica and with the silica coupler to thereby render a more efficient reaction of the precipitated silica and coupler in the second phase (comprised of the cis 1,4-polyisoprene rubber and precipitated silica with silica coupler in the absence of the zinc oxide).
The fatty acid (e.g. carboxylic acid such as, for example, at least one of stearic, palmitic and oleic acid) is to be selectively added in the first phase (polybutadiene rubber with carbon black reinforcement) to promote better processing of the polybutadiene rubber followed by promoting better processing of the combination of the rubber phases, taking into consideration that it is envisioned that the natural rubber naturally contains a small amount (e.g. from about 0.5 to about 1.2 phr) of fatty acid.
The zinc oxide and fatty acid are added together in the first phase (polybutadiene rubber with carbon black reinforcement) to promote formation of fatty acid salt (e.g. zinc stearate) within the polybutadiene rubber of the first phase to thereby promote activation of subsequently added sulfur cure accelerators (sulfur cure accelerators added to the subsequently blended first and second rubber phases) predominately in the presence of the polybutadiene rubber.
The two rubber phases (masterbatches) are to be mixed together followed by mixing with sulfur curatives or mixed together with sulfur curatives.
A silica coupling agent may be mixed together with the precipitated silica in the second masterbatch or added after the two masterbatches are mixed together, although it is usually desired to blend the silica coupler with the precipitated silica containing second masterbatch.
In practice, it is recognized that a reinforcing filler (e.g. the rubber reinforcing carbon black and precipitated silica) has a preferential reinforcing affinity for the first diene-based elastomer with which it is mixed (e.g. the carbon black for the cis 1,4-polybutadiene rubber in the first phase or the precipitated silica with the natural rubber in the second phase, particularly when combined with the coupling agent in the second phase). It is believed that such preferential reinforcing filler reinforcement phenomenon is understood by those having skill in such art.
For this evaluation, it is therefore desired to individually and separately promote such preferential reinforcing affinities of the reinforcing fillers with the respective elastomers by the preparation of the two rubber phases (two masterbatches) followed by blending the phases together.
In this manner, in the blended elastomer composition, it is envisioned that the carbon black maintains a greater preferential reinforcing affinity for the cis 1,4-polybutadiene rubber of the first rubber phase and the precipitated silica maintains a greater preferential reinforcing affinity for the natural rubber of the second rubber phase, all within the blended elastomer composition (blended masterbatches).
As a result, it is envisioned that carbon black presents a comparatively less reinforcing effect for the natural rubber of the second phase and the precipitated silica has a comparatively less reinforcing effect for the cis 1,4-polybutadiene rubber of the first phase.
As a result, for the blend of the rubber phases (masterbatches), it is further envisioned that only a limited migration of the carbon black into the natural rubber of the second phase may occur and only a limited migration of the precipitated silica into the cis 1,4-polybutadiene rubber of the first phase may occur.
Therefore, it is envisioned that the resultant rubber composition is largely composed of two phases, namely a carbon black reinforced cis 1,4-polybutadiene rubber phase and a precipitated silica reinforced natural rubber phase.
In this manner a combination of predominantly carbon black reinforced cis 1,4-polybutadiene rubber properties may be promoted and predominantly precipitated silica reinforced natural rubber properties may be promoted for the resultant tread rubber composition.
In one embodiment, it is envisioned that a dominating rubber phase (e.g. a continuous phase) is the first phase comprised of cis 1,4-polybutadiene rubber containing the rubber reinforcing carbon black in the instance where the first phase is of a larger concentration in the rubber composition than the second phase (e.g. a dispersed phase within the continuous phase), comprised of a natural rubber-containing the precipitated silica.
As indicated, the individual first and second rubber phases may, for convenience, also be respectively referred to as first and second rubber masterbatches.
The rubber composition comprised of the blend of the individual rubber phases, or masterbatches, may then be shaped to form a tire tread and sulfur cured, together with assembled tire components, to form the tire.
It is important to appreciate that such preparation of the dual phased rubber composition for this evaluation is intended to be a significant departure from a more simple step-wise sequential mixing of precipitated silica and rubber reinforcing carbon black reinforcing fillers in a rubber composition and from a more simple dual masterbatch mixing of elastomers and reinforced fillers.
The term “phr” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer”. The terms “rubber” and “elastomer” where used herein, are to be used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “vulcanize” and “cure” where used therein are used interchangeably unless otherwise indicated.